A heating, ventilating and air conditioning system (hereinafter also being referred to as HVAC) may consist of equipment to condition air as needed (to heat, cool, dry, moisten or purify it, etc.), and then to force the conditioned air to a zone, in order to satisfy demand called for in that zone. Each system may include at least two air-flow diffusers in the zone, one for the discharge of air to the zone and one for the return of air from the zone. Ducts may connect each diffuser to the HVAC equipment located remotely of the zone. A sensor, such as a thermostat for temperature, may detect demand in the zone, to operate the HVAC unit for such zone responsive to such demand.
Certain HVAC systems may have multiple units, for conditioning multiple zones; each unit being the duplicate of the other. Each unit would primarily be responsible for its one zone, and would be cycled on and off responsive to the demand of its zone.
Under many outdoor ambient temperatures, the design capacity of the HVAC unit far exceeds the conditioning output needed to satisfy the demand of the zone. Under such operating conditions, each unit will be cycled on and off, sometimes quite frequently and/or for only short durations, responsive to the instantaneous zone demands.
Operating HVAC equipment under frequent short cycles is hard on the equipment, possibly leading to more frequent maintenance demands and/or break-downs than equipment run over sustained periods. Moreover, the efficiency drops off, as the equipment itself initially must be run merely to bring its own hardware to operating temperatures, at the start of the operating cycle; and the energies of the cycle may merely be dissipated without reaching the conditioned zone, at the end of the cycle. Reduced efficiency means more total overall operating hours for such equipment, to yield the same needed thermal output.
Additionally, with a multiple unit HVAC system, should two or more of the HVAC units come on simultaneously, the allowable instanteous peak utility demand, set by the local utility company, may be exceeded . . . to result then in a surcharge being imposed on the utility billing rate. Load sheading controls are available to avoid this; but such generally are costly and complicated, and utilize high-tech solid state systems and/or dedicated computers. Many users now have only standard HVAC equipment of a hands-on familiarity, and may conscientiously or unconscientiously avoid that initial step into the complexities and unknowns of the high-tech world . . . including the known elevated installation and service charges.